Ring mills in the form of roller mills or ball ring mills are well known. In these mills the grinding balls or rollers roll on a circular grinding plate, and the grinding pressure is produced by spring tension, hydraulic cylinders, centrifugal force, or by the weight of the balls or rollers.
In mills of this type a stationary nozzle ring mounted on the outer periphery of the grinding plate delivers an air stream which picks up the fine constituents of the comminuted material discharged over the edge of the grinding plate and carries them upwards, while the coarse constituents fall downward through the nozzle ring against the stream of air and are returned to the mill by means of a mechanical elevator for example.
In the past ring mills of this type were only used for the production of a relatively fine-grained finished product. This finished product was discharged from the mill with the air stream and separated off in separators (such as cyclones or filters).
There is now an occasional requirement in the art to produce finished products of differing fineness. For instance, in the production of aggregates a fine product, such as filler (with a grain size less than 0.2 mm), and a coarse product, such as an addition to masonry cement (with a grain size of 0 to 4 mm) may be required. In the past such finished products of differing fineness generally were produced in separate grinding apparatus, which involves a considerable expenditure on plant.
The object of the invention, therefore, is to provide grinding apparatus and a process which enables at least two finished products of differing fineness to be produced simultaneously in a simple and economical manner.